Silane coupling agents have previously been used with significant success to chemically couple various resins to mineral filler materials such as glass and thereby upgrade the mechanical properties of glass reinforced epoxy and unsaturated polyester plastics. Although a great many of these well established coupling agents have been effective in coupling glass or other mineral fillers to resins such as nylon, they have been largely ineffective in coupling such fillers to less reactive thermoplastics such as polyolefins and thermoplastic polyesters. The lack of effectiveness of the traditional coupling agents in coupling polyolefins and thermoplastic polyesters to mineral filler materials is believed due to the inability of such couplers to chemically bond to these resins. Previous attemps to resolve this problem have included the use of special azido compounds such as those described in U.S. Pat. No. 3,766,216 to bond unreactive polymers to ethylenically unsaturated hydrocarbon silane treated glass filler. This of course requires the use of a conventional silane coupling agent in addition to the special azido compounds of the above mentioned patent.
The advantages of reinforcing polybutylene terephthalate (PBT) with mineral filler such as glass spheres or fibers has been recognized for some time. The use of glass fibers in PBT has been described for instance in U.S. Pat. No. 3,814,725. Unfortunately, no coupling agents have previously been known to be effective in satisfactorily coupling PBT to mineral fillers.